It is known that the separation and recycle or otherwise retaining solids containing anaerobic bacteria will provide significant benefits during the anaerobic conversion of sewage or waste water streams to soluble and gaseous products. See U.S. Pat. No. 5,015,384 to the inventor of the subject application. Also, see U.S. Pat. No. 5,514,277 to Boris M. Khudenko. The separation and recycle process preserves the bacteria responsible for conversion of biodegradable organic material to gaseous and soluble products and maintains slowly metabolized solids for prolonged periods of time within an anaerobic reactor where the slowly metabolized solids can be effectively digested.
The recycle processes described in the two patents noted above are limited by the accumulation of solids within the anaerobic reactor. The accumulation can be countered by wasting such solids from the anaerobic reactor. Unfortunately, the wasting of solids from the anaerobic reactor not only removes inorganic materials and refractory organic materials (organic materials not digested in a timely manner), but also removes the desirable biomass and also biodegradable organic material that has yet to be fully digested. By wasting solids from the anaerobic reactor, the solids retention time is reduced which limits the maximum potential of the recycle system since conversion of the biodegradable organic material to soluble and gaseous products is directly related to the solids retention time.
The build-up of solids within the anaerobic treatment process also adversely affects the completeness of mixing and heat transfer that can occur in the anaerobic reactors because of the higher solids concentration. As mixing and heat transfer efficiency decreases, the effectiveness of the anaerobic digestion of the biodegradable organic materials is reduced. Additionally, as the solids concentration increases, it becomes more difficult and costly to separate biodegradable components from nonbiodegradable components. For example, as the solids concentration increases, more energy and chemical conditioning is required to drive water from the solids. This action can be counterproductive to bacterial viability and process performance. For example, there is a direct relationship between the energy used in centrifuge separations of yeast and bacteria cells and the centrate SCOD resulting from cell rupture. The efficiency of membrane separation is also a function of the solids content of the feed stream, or retentate, since it has considerable influence on the permeate rate and its purity.